Studies on the audiogenic seizure-shock episode in weanling rats with acute magnesium deficiency have been directed toward the pulmonary and cardiac ultrastructural changes which have revealed aggregates of platelets, leukocytes, erythrocytes and lymphocytes in the capillaries, with occasional reticulocytes. Increased release of thromboxane A2 (TxA2), a platelet aggregator, is associated with magnesium deficiency. A specific TxA2 receptor antagonist at sufficiently high doses almost completely aborted the seizure-shock episode in rats, and the animals (and their lungs) were normal. Histamine was ruled out as a contributing factor in the seizure- shock episode. A histamine antagonist did not abort the seizure- shock episode, and although plasma histamine from unstressed Mg deficient rats was elevated over control levels, it did not further increase during seizure-shock. The kidney in furosemide-treated Mg deficient weanling rat have shown high levels of calcium. The kidney was studied in adult rats. treated with furosemide (Lasix). Marked renal tubular calcification was demonstrated biochemically and histologically in moderately deficient rats (10 mg magnesium/100 8). Rats fed the National Research Council's Recommended 40 mg/100 g diet showed sporadic elevations of renal calcium. The calciuric effect of furosemide was blocked at higher levels of dietary magnesium, with normal renal calcium levels. The parenteral Mg load test provides and evaluation of the Mg stores; we evaluated the relationship between parenteral Mg load retention to the young adult rat's dietary, plasma, and bone magnesium. The relationship between the logarithm of percent retention and plasma or femur magnesium level was approximated by a decreasing straight line. Plasma and femur concentrations of magnesium varied linearly. This is a model for the magnesium retention test for young adult humans.